ConductorMethods

Class that facilitates the testing of classes, traits, and libraries designed to be used by multiple threads concurrently.

A Conductor conducts a multi-threaded scenario by maintaining a clock of "beats." Beats are numbered starting with 0. You can ask a Conductor to run threads that interact with the class, trait, or library (the subject) you want to test. A thread can call the Conductor's waitForBeat method, which will cause the thread to block until that beat has been reached. The Conductor will advance the beat only when all threads participating in the test are blocked. By tying the timing of thread activities to specific beats, you can write tests for concurrent systems that have deterministic interleavings of threads.

A Conductor object has a three-phase lifecycle. It begins its life in the setup phase. During this phase, you can start threads by invoking the thread method on the Conductor. When conduct is invoked on a Conductor, it enters the conducting phase. During this phase it conducts the one multi-threaded scenario it was designed to conduct. After all participating threads have exited, either by returning normally or throwing an exception, the conduct method will complete, either by returning normally or throwing an exception. As soon as the conduct method completes, the Conductor enters its defunct phase. Once the Conductor has conducted a multi-threaded scenario, it is defunct and can't be reused. To run the same test again, you'll need to create a new instance of Conductor.

Here's an example of the use of Conductor to test the ArrayBlockingQueue class from java.util.concurrent:

import org.scalatest.fixture.FunSuite
import org.scalatest.matchers.Matchers
import java.util.concurrent.ArrayBlockingQueue
import org.scalatest.concurrent.Conductors

class ArrayBlockingQueueSuite extends FunSuite with Matchers with Conductors {

 test("calling put on a full queue blocks the producer thread") {

   val conductor = new Conductor
   import conductor._

   val buf = new ArrayBlockingQueue[Int](1)

   thread("producer") {
     buf put 42
     buf put 17
     beat should be (1)
   }

   thread("consumer") {
     waitForBeat(1)
     buf.take should be (42)
     buf.take should be (17)
   }

   whenFinished {
     buf should be ('empty)
   }
 }
}

When the test shown is run, it will create one thread named producer and another named consumer. The producer thread will eventually execute the code passed as a by-name parameter to thread("producer"):

buf put 42
buf put 17
beat should be (1)

Similarly, the consumer thread will eventually execute the code passed as a by-name parameter to thread("consumer"):

waitForBeat(1)
buf.take should be (42)
buf.take should be (17)

The thread calls create the threads and starts them, but they will not immediately execute the by-name parameter passed to them. They will first block, waiting for the Conductor to give them a green light to proceed.

The next call in the test is whenFinished. This method will first call conduct on the Conductor, which will wait until all threads that were created (in this case, producer and consumer) are at the "starting line", i.e., they have all started and are blocked, waiting on the green light. The conduct method will then give these threads the green light and they will all start executing their blocks concurrently.

When the threads are given the green light, the beat is 0. The first thing the producer thread does is put 42 in into the queue. As the queue is empty at this point, this succeeds. The producer thread next attempts to put a 17 into the queue, but because the queue has size 1, this can't succeed until the consumer thread has read the 42 from the queue. This hasn't happened yet, so producer blocks. Meanwhile, the consumer thread's first act is to call waitForBeat(1). Because the beat starts out at 0, this call will block the consumer thread. As a result, once the producer thread has executed buf put 17 and the consumer thread has executed waitForBeat(1), both threads will be blocked.

The Conductor maintains a clock that wakes up periodically and checks to see if all threads participating in the multi-threaded scenario (in this case, producer and consumer) are blocked. If so, it increments the beat. Thus sometime later the beat will be incremented, from 0 to 1. Because consumer was waiting for beat 1, it will wake up (i.e., the waitForBeat(1) call will return) and execute the next line of code in its block, buf.take should be (42). This will succeed, because the producer thread had previously (during beat 0) put 42 into the queue. This act will also make producer runnable again, because it was blocked on the second put, which was waiting for another thread to read that 42.

Now both threads are unblocked and able to execute their next statement. The order is non-deterministic, and can even be simultaneous if running on multiple cores. If the consumer thread happens to execute buf.take should be (17) first, it will block (buf.take will not return), because the queue is at that point empty. At some point later, the producer thread will execute buf put 17, which will unblock the consumer thread. Again both threads will be runnable and the order non-deterministic and possibly simulataneous. The producer thread may charge ahead and run its next statement, beat should be (1). This will succeed because the beat is indeed 1 at this point. As this is the last statement in the producer's block, the producer thread will exit normally (it won't throw an exception). At some point later the consumer thread will be allowed to complete its last statement, the buf.take call will return 17. The consumer thread will execute 17 should be (17). This will succeed and as this was the last statement in its block, the consumer will return normally.

If either the producer or consumer thread had completed abruptbly with an exception, the conduct method (which was called by whenFinished) would have completed abruptly with an exception to indicate the test failed. However, since both threads returned normally, conduct will return. Because conduct doesn't throw an exception, whenFinished will execute the block of code passed as a by-name parameter to it: buf should be ('empty). This will succeed, because the queue is indeed empty at this point. The whenFinished method will then return, and because the whenFinished call was the last statement in the test and it didn't throw an exception, the test completes successfully.

This test tests ArrayBlockingQueue, to make sure it works as expected. If there were a bug in ArrayBlockingQueue such as a put called on a full queue didn't block, but instead overwrote the previous value, this test would detect it. However, if there were a bug in ArrayBlockingQueue such that a call to take called on an empty queue never blocked and always returned 0, this test might not detect it. The reason is that whether the consumer thread will ever call take on an empty queue during this test is non-deterministic. It depends on how the threads get scheduled during beat 1. What is deterministic in this test, because the consumer thread blocks during beat 0, is that the producer thread will definitely attempt to write to a full queue. To make sure the other scenario is tested, you'd need a different test:

test("calling take on an empty queue blocks the consumer thread") {

 val conductor = new Conductor
 import conductor._

 val buf = new ArrayBlockingQueue[Int](1)

 thread("producer") {
   waitForBeat(1)
   buf put 42
   buf put 17
 }

 thread("consumer") {
   buf.take should be (42)
   buf.take should be (17)
   beat should be (1)
 }

 whenFinished {
   buf should be ('empty)
 }
}

In this test, the producer thread will block, waiting for beat 1. The consumer thread will invoke buf.take as its first act. This will block, because the queue is empty. Because both threads are blocked, the Conductor will at some point later increment the beat to 1. This will awaken the producer thread. It will return from its waitForBeat(1) call and execute buf put 42. This will unblock the consumer thread, which will take the 42, and so on.

The problem that Conductor is designed to address is the difficulty, caused by the non-deterministic nature of thread scheduling, of testing classes, traits, and libraries that are intended to be used by multiple threads. If you just create a test in which one thread reads from an ArrayBlockingQueue and another writes to it, you can't be sure that you have tested all possible interleavings of threads, no matter how many times you run the test. The purpose of Conductor is to enable you to write tests with deterministic interleavings of threads. If you write one test for each possible interleaving of threads, then you can be sure you have all the scenarios tested. The two tests shown here, for example, ensure that both the scenario in which a producer thread tries to write to a full queue and the scenario in which a consumer thread tries to take from an empty queue are tested.

Class Conductor was inspired by the MultithreadedTC project, created by Bill Pugh and Nat Ayewah of the University of Maryland.

Although useful, bear in mind that a Conductor's results are not guaranteed to be accurate 100% of the time. The reason is that it uses java.lang.Thread's getState method to decide when to advance the beat. This use goes against the advice given in the Javadoc documentation for getState, which says, "This method is designed for use in monitoring of the system state, not for synchronization." In short, sometimes the return value of getState occasionally may be inacurrate, which in turn means that sometimes a Conductor could decide to advance the beat too early. In practice, Conductor has proven to be very helpful when developing thread safe classes. It is also useful in for regression tests, but you may have to tolerate occasional false negatives.

Configuration object for asynchronous constructs, such as those provided by traits Eventually and Waiters.

The default values for the parameters are:

Configuration Parameter	Default Value
timeout	scaled(150 milliseconds)
interval	scaled(15 milliseconds)

Gets the current value of the clock. Primarily useful in assert statements.

Check if the clock has been frozen by any threads. (The only way a thread can freeze the clock is by calling withClockFrozen.)

Create a new thread that will execute the given function. If the test is started, then the thread will run the function immediately. If it is not yet started, the Thread will wait to run the function until all threads are up and ready to go.

Create a new thread that will execute the given function. If the test is started, then the thread will run the function immediately. If it is not yet started, the Thread will wait to run the function until all threads are up and ready to go.

Force the current thread to block until the thread clock reaches the specified value, at which point the current thread is unblocked.

Register a function to be executed after the simulation has finished.

Run the passed function, ensuring the clock does not advance while the function is running (has not yet returned or thrown an exception).

Creates and initializes a private instance variable with a new Conductor, ensuring it is visible to any thread, invokes the passed test function, and invokes conduct on the Conductor, if it was not already invoked by the test.

This trait is stackable with other traits that override withFixture(NoArgTest), because instead of invoking the test function directly, it delegates responsibility for invoking the test function to super.withFixture(NoArgTest).

The total number of tests that are expected to run when this Suite's run method is invoked.

Returns an Interval configuration parameter containing the passed value, which specifies the amount of time to sleep after a retry.

An immutable IndexedSeq of this SuiteMixin object's nested Suites. If this SuiteMixin contains no nested Suites, this method returns an empty IndexedSeq.

The fully qualified name of the class that can be used to rerun this suite.

Runs this suite of tests.

Runs zero to many of this suite's nested suites.

Runs a test.

Runs zero to many of this suite's tests.

Scales the passed Span by the Double factor returned by spanScaleFactor.

The Span is scaled by invoking its scaledBy method, thus this method has the same behavior: The value returned by spanScaleFactor can be any positive number or zero, including a fractional number. A number greater than one will scale the Span up to a larger value. A fractional number will scale it down to a smaller value. A factor of 1.0 will cause the exact same Span to be returned. A factor of zero will cause Span.ZeroLength to be returned. If overflow occurs, Span.Max will be returned. If underflow occurs, Span.ZeroLength will be returned.

The factor by which the scaled method will scale Spans.

The default implementation of this method will return the span scale factor that was specified for the run, or 1.0 if no factor was specified. For example, you can specify a span scale factor when invoking ScalaTest via the command line by passing a -F argument to Runner.

A string ID for this Suite that is intended to be unique among all suites reported during a run.

The suite ID is intended to be unique, because ScalaTest does not enforce that it is unique. If it is not unique, then you may not be able to uniquely identify a particular test of a particular suite. This ability is used, for example, to dynamically tag tests as having failed in the previous run when rerunning only failed tests.

A user-friendly suite name for this Suite.

This trait's implementation of this method returns the simple name of this object's class. This trait's implementation of runNestedSuites calls this method to obtain a name for Reports to pass to the suiteStarting, suiteCompleted, and suiteAborted methods of the Reporter.

A Map whose keys are String names of tagged tests and whose associated values are the Set of tag names for the test. If a test has no associated tags, its name does not appear as a key in the returned Map. If this Suite contains no tests with tags, this method returns an empty Map.

Subclasses may override this method to define and/or discover tags in a custom manner, but overriding method implementations should never return an empty Set as a value. If a test has no tags, its name should not appear as a key in the returned Map.

Provides a TestData instance for the passed test name, given the passed config map.

This method is used to obtain a TestData instance to pass to withFixture(NoArgTest) and withFixture(OneArgTest) and the beforeEach and afterEach methods of trait BeforeAndAfterEach.

A Set of test names. If this Suite contains no tests, this method returns an empty Set.

Although subclass and subtrait implementations of this method may return a Set whose iterator produces String test names in a well-defined order, the contract of this method does not required a defined order. Subclasses are free to implement this method and return test names in either a defined or undefined order.

Returns a Timeout configuration parameter containing the passed value, which specifies the maximum amount to wait for an asynchronous operation to complete.

The styleName lifecycle method has been deprecated and will be removed in a future version of ScalaTest.

This method was used to support the chosen styles feature, which was deactivated in 3.1.0. The internal modularization of ScalaTest in 3.2.0 will replace chosen styles as the tool to encourage consistency across a project. We do not plan a replacement for styleName.

Implicit PatienceConfig value providing default configuration values.

To change the default configuration, override or hide this def with another implicit PatienceConfig containing your desired default configuration values.